In the malaria parasite, Plasmodium falciparum, resistance to drugs that inhibit dihydrofolate reductase (DHFR) results from point mutations in the DHFR gene. We have engineered yeast whose growth depends upon the DHFR enzyme from drug-sensitive P. falciparum, and shown that these yeast are killed by antimalarial DHFR inhibitors at concentrations in the therapeutic range. This is a collaborative project initiated with Jacobus Pharmaceutical, Inc., manufacturers of a new class of biguanide antifols. The first of these drugs, PS-15, is effective even against alleles of Pf-DHFR resistant to all currently test drugs of this class. The Sibley lab will use yeast that depend upon P. falciparum DHFR as rapid system to screen newly synthesized analogs of the activated triazine form of PS- 15 called WR-99210. We will use saturation mutagenesis of the PF- DHFR to identify any mutations in DHFR that can confer resistance to each drug, Jacobus Pharmaceutical will synthesize derivatives of the triazine (WR99210), based on the structure activity relationships identified in the initial yeast screen. This yeast system is a rapid, inexpensive system to identify drugs or combinations to which resistant mutants are selected most slowly. We will test the hypothesis that mutations that confer resistance to WR-99210 encode an enzyme that is more sensitive to currently used antifolate drugs like pyrimethamie. The goal is to gather the basic information necessary to move into animal and finally human trials so that this promising new antimalarial drug can be brought to market. The system is a prototype; it can be modified to screen drugs directed against other parasite targets in P. falciparum, the related human parasites, Toxoplasma, Pneumocystis, Cryptosporidium or the agricultural parasites, Eimeria, Theileria or Babesia. PROPOSED COMMERCIAL APPLICATION: Information will be generated which will greatly decrease the risk of quick development of resistance to this drug series in the field and will also provide insight into which combinations of drugs from this series with other DHFR inhibitors are least likely to be lead to the quick development of resistance strains of malaria. This will greatly increase the value of the drug and therefore its attractiveness for commercial development.